The etching of silicon has been widely applied in the semiconductor industry. Common forms of silicon etching are by plasma etch and wet chemical etch. Plasma etching enables fine features to be formed in the silicon, but is relatively expensive. Wet chemical etching is less expensive, but produces coarser features.
In some applications, a silicon wafer is etched from the back surface with a wet chemical etch process such that some portions of the silicon extending from the back surface to the front surface are removed. In these processes, a layer of silicon dioxide (SiO.sub.2) or silicon nitride (Si.sub.3 N.sub.4) is usually formed over the front surface of the silicon wafer as an etch stop. Typical silicon etchants attack these materials at a much slower rate than silicon.
Silicon dioxide and silicon nitride layers work adequately as long as the removed portions of silicon are relatively small. However, these materials have been found to crack and break apart when larger portions of silicon are removed, which is often catastrophic to the overall fabrication process. As is known in the art, the underlying silicon wafer places a significant amount of tangential stress on silicon dioxide and nitride layers which are formed over its surface. Oxide and nitride layers (SiO.sub.2 and Si.sub.3 N.sub.4) are formed over the silicon wafer by high temperature processes (usually&gt;700.degree. C.), and have coefficients of thermal expansion (CTE) which are smaller than that of silicon. Upon cooling to room temperature, the oxide and nitride layers prefer to contract at a slower rate than the underlying silicon wafer. However, because the silicon wafer is thicker than these layers, the silicon wafer determines the overall lateral contraction rate and thereby compresses these layers in the tangential direction. When the silicon under a large area of a SiO.sub.2 or Si.sub.3 N.sub.4 layer is etched away, the tangential stress in the layer causes the layer to bow outwards in the vertical direction. With sufficient bowing, the layer cracks or breaks.
Accordingly, there is a need for an etch-stop layer that can be used for the removal of large portions of silicon.